Low melting temperature alloy structure for enchanced thermal interface

ABSTRACT

A method and associated apparatus for providing an enhanced thermal interface. The interface is formed by application of a structure or foil embedded in an alloy in solid form between two surfaces. Once heat is applied from one or both surfaces, the alloy melts forming the desired interface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus for providing an enhanced thermal interface; and more particularly for a method and apparatus for providing an enhanced thermal interface used in semiconductor packaging of a computing environment.

2. Description of Background

The evolution of semiconductor industry had led to an industry trend of continuously increasing the number of electronic components inside an electronic device. Compactness allows for selective fabrication of smaller and lighter devices that are more attractive to the consumer. In addition, compactness also allows many of the circuits to operate at higher frequencies and at higher speeds due to shorter electrical distances in these devices. Despite many of the advantages associated with this industry goal, providing many such components in a small footprint create device performance challenges. One such challenge has to do with creating thermal interfaces that do not interfere with electrical conductivity and integrity of the device.

In order to provide a good interface, all surfaces involved have to be clean, smooth and free of all particles including air gaps or bubbles that may be formed during manufacturing process. Heat and mechanical forces exerted during normal and continuous device operation have to also be considered as they may affect the integrity of the interface. In this regard, the interface has to be fabricated in a manner that it does not degrade easily after continuous use and special attention must be given to the possibility that any such degradation or related problems may potentially cause electrical shorts or other such similar problems that affect device operation and integrity as a whole.

One of the more challenging interfaces to fabricate and maintain are thermal interfaces, such as those used in a computing environments for example between microprocessor chips and heat sinks. In such instances, interface joint needs to be able to provide the needed heat transfer and dissipation that is crucial to the speed and power capabilities of the circuit, as well as meeting the requirements of remaining gaps free and other as discussed.

Prior art has used several methods to provide improved thermal interfaces. One way is to use thermal paste. Thermal paste has been used to fill the gaps between the back sides of chips to provide such interface. Using thermal paste, however, has multiple problems associated with it. For one assembly of devices with a thermal paste component is difficult as sufficient paste must be dispensed to completely cover the chip. In addition, thermal paste is viscous and difficult to handle. Module components uses must also be chemically compatible with the thermal paste to provide good adherence. Finally, the paste filled chip must have sufficient thickness that it will form a reliable structure.

Another attempt to provide a good thermal interface is by direct application of alloys between joints. Metallic bonds are a preferred thermal joint because they have very little thermal resistance. Metals, as a class, exhibit high thermal conductivity. Metals in liquid state exhibit high thermal conductivity and have the added advantage of being able to fill the voids in mechanical joints. Nonetheless, the same features that make these joints attractive, can also lead to device integrity problems.

The problem is often caused due to poor wetting inherent in the low-melting temperature alloy interfaces between microprocessor chips and heat sinks. The problem occurs due to the fact that when a layer of low-melting alloy is melted while it is squeezed between two surfaces, capillary action due to poor wetting, between the low-melting alloy and the chip and between the low-melting alloy and the heat sink, forces the molten alloy out of the interface. With no molten alloy bridging the gap, the thermal performance of the cooling assembly suffers. The molten alloy that is forced out of the interface, can also ooze and leak to other, and especially adjacent areas housing electrical components and potentially cause system degradation and failure.

Consequently, it is desired to design a method and apparatus that can be used to provide good thermal interfaces while taking advantage of the benefits offered by using molten alloys in such interfaces.

SUMMARY OF THE INVENTION

The shortcomings of the prior art are overcome and additional advantages are provided through the method and associated apparatus for providing an enhanced thermal interface. The to be formed interface is preferably between a heat sink and a chip and the operation of the chip provides the required heat dissipation needed to melt the alloy. The interface is formed by application of a structure or foil embedded in an alloy in solid form between two surfaces. Once heat is generated, such as through normal device operation, the alloy in the foil or structure melts forming the desired interface. The structure and foil, however, prevents the leaking and oozing of the alloy to surrounding areas. In alternate embodiments, the structure can have apertures such as a wire mesh to provide certain amount of rigidity and compressibility.

Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross sectional illustration of an enhanced thermal joint;

FIG. 2 is a cross sectional illustration of one embodiment of the present invention using a wire mesh structure in forming the thermal interface of the embodiment of FIG. 1; and

FIG. 3 is an alternate embodiment of the present invention using an alloy embedded foil in forming the thermal interface of the embodiment on FIG. 1.

DESCRIPTION OF THE INVENTION

FIG. 1 provides a cross sectional illustration of an enhanced thermal interface. As illustrated in FIG. 1, a first and a second surface respectively referenced as 110 and 120 is provided between which a thermal interface is to be formed. In a preferred embodiment of the present invention the surfaces 110 and 120 are the chip and heat sink components of a computing environment, but this is not a requirement.

Thereafter the thermal interface is formed and depicted as illustrated and referenced at 130 between the surface 110 and 120. The thermal interface 130 is comprised preferably of a metal alloy. As illustrated no air gaps or other surface irregularities are present and the thermal interface 130 has been confined to the area limited to the first and second surfaces 10 and 120 without any portion of the interface 130 having oozed out or leaked out from any of its sides to its surroundings and/or adjacent areas.

The interface 130, as will be discussed below, can be first formed by application of the alloy in solid form. When appropriate material is chosen for the interface alloy, heat dissipated from the computing environment (or alternatively first and second component 110 and 120) will melt the alloy, causing the resultant smooth thermal interface 130 as illustrated in FIG. 1.

Heat dissipation is an important challenge for the designers of computing environment, and especially those that involve large and complex environments. Heat dissipation if unresolved, however, can result in electronic and mechanical failures that will affect overall system performance, no matter what the size of the environment. As can be easily understood, the heat dissipation increases as the packaging density increases. In larger computing system environments, not only the number of heat generating electronic components are more numerous than that of smaller environments, but thermal management solutions must be provided that take other needs of the system environment into consideration as heat dissipation can create a variety of other seemingly unrelated problems. That is why thermal interfaces, particularly, are of such concern in these environments.

In fact, the efficient extraction of heat from integrated circuit packages used in any computing environment has presented a very significant limitation on the design capability of these circuits. Whenever it is desired to transfer or dissipate heat from one object to another by conduction, the critical element is the maintenance of the joint between the surfaces.

Efficient removal of heat from a heat producing device such as a microprocessor silicon chip requires that it be in intimate contact with a heat dissipating device such as a heat sink. In such interfaces, however, both the smoothness of the surface and even sometimes some measure of pressure that needs to be applied to the joint continuously may be key in maintaining a good joint.

It is possible, in this regard, to maintain mating surfaces flat and apply pressure to hold them together. Unfortunately no matter however microscopic are the surface irregularity, such irregularities combined with manufacturing process steps create air gaps between the chip and the heat sink that severely degrades the heat transfer performance. Known materials that can enhance such joints are pastes and greases that are either difficult to apply, or can reduce the thermal resistance of the contacting surfaces as discussed.

Another known solution to create smooth joint surfaces that are free of air gaps and other irregularities is to fill such gaps with low-melting alloys that could be liquid or even solid under normal operating temperature. The problem is that when this low-melting alloy melts to conform to the gap, if there is poor wetting between the low-melting alloy and the gap surfaces, the surface tension will force the liquid alloy out of the gap.

It is not easy to have the interface surfaces be very clean and easily wetted by the low-melting alloy. Also, a foil of the low-melting alloy that could be placed at the interface has surface oxide build up on it during storage. Not perfectly clean interface surfaces and oxide on the low-melting alloy foil cause poor wetting by the low-melting alloy. Therefore, there is a need for a means that will keep the low-melting alloy in its intended place, filling the interface gap and providing a thermal bridge between chip and the heat sink as discussed earlier.

The present invention provides for a means of bridging the gap between a chip and a heat sink by a low-melting alloy even when the low-melting alloy does not wet the chip and the heat sink. In the absence of good wetting, the capillary forces, would force the molten alloy to exit the gap. The present invention provides means by which the low-melting alloy stays in the gap even when the wetting is very poor. The exact structure of the alloy 130 can be further explored by means of examples as provided in conjunction with FIGS. 2 and 3 below.

It should be noted, that while a variety of different embodiments can be achieved as can be appreciated by those skilled in the art, a couple of such embodiments are provided in FIGS. 2 and 3 below with the understanding that these are only provided for discussion purposes and therefore are not limiting as to the subject matter of the present invention. Two such means will now be explored in greater details.

FIG. 2, is a cross sectional illustration of a first embodiment of the present invention. In FIG. 2, a first embedded structure 200 is illustrated. The structure is preferably comprised of metal and has a form, such as being apertured, so that it can maintain a level of structural integrity and rigidity while being able to become embedded in an alloy 210 as will be discussed.

Despite some desired structural rigidity, in some instances it may be required for the interface and the structure 200 involved to maintain certain level of flexibility and compressibility. In this regard, it should be noted that the level of rigidity (or even compressibility) can be selectively achieved, maintained or increased by increasing or decreasing the level of surface apertures. In one embodiment of the present invention, the structure 200 has a mesh like structure to provide both rigidity and compressibility at the same time. In a preferred embodiment, the structure 200 is a wire mesh that it can easily contain alloy 210 as illustrated.

The alloy 210 used is preferably a low melting alloys. In addition, the materials used for the low-melting alloy and the apertured structure 200 have to be such that they do not readily react with one another.

As illustrated in the preferred embodiment of FIG. 2 where a wired mesh (200) is utilized, the wire mesh is embedded in the low-melting alloy 210. As discussed earlier, any material that can hold the alloy in the molten form and be embedded in it, can be used as well as a wire mesh.

In a preferred embodiment, before the wire mesh or other such material is embodied in the alloy, it is well cleaned by means known to those skilled in the art. In one embodiment, the wire mesh is cleaned by sputter etching before the application of the low-melting alloy.

It should be noted that the thickness of the alloy embedded structure (200) depends on how much metal alloy is applied to it and can be selectively altered to suit different purposes. The low melting alloy can also be applied by a variety of methods known to those skilled in the art. Metal alloy, for example can be sputtered on the wire mesh or other such structure, or alternatively be deposited on the wire mesh using chemical or physical vapor deposited techniques. The alloy should at least be applied to two sides 201 and 202 of the structure 200, but it should be preferably deposited on all sided of the structure 200, including in between apertures, when the structure 200 provides such apertures in its design.

Referring back to FIG. 1, the embedded structure 200 of FIG. 2 is then applied in between the two surfaces 110 and 120, preferably in solid or substantially solid form. As discussed earlier, the heat dissipated from the computing environment, or the surfaces when such is not the case, during normal device operation resulting in heat dissipation that in turn melts the alloy and form a gap free, bubble free interface 130 of FIG. 1.

The structural rigidity of the structure 200 will hold the molten alloy in place and prevents it from the effects of the capillary action due to poor wetting as discussed earlier. In this regard, it should be noted that the structural rigidity of the structure 200 also provides ease of applicability. The process of fabricating the structure 200 is not cost prohibitive and flexible. This fabrication process can be easily integrated in the fabrication process of other semiconductor components that will be used in the environment. Therefore existing manufacturing processes and components can be used in its formation.

FIG. 3 provides for an alternate embodiment of the present invention. In FIG. 3, a cross sectional illustration of a second embodiment for forming the interface 130 of FIG. 1 is provided using a foil 300. The foil 300 is embedded in a low-melting alloy 310 as depicted. As before the material of the foil 300 and the low melting alloy 310 have to be such that they do not readily react with one another.

In a preferred embodiment, before the foil 300 is embodied in the alloy 310, it is well cleaned by means known to those skilled in the art. To do so, in a preferred embodiment, the foil 300 is cleaned by sputter etching as before.

The thickness of the alloy embedded structure (300) can be selectively altered. In this case, the thickness can be a combination of thickness of the foil 300 or the applied alloy 310. The low melting alloy can also be applied by a variety of methods known to those skilled in the art as discussed before. For example, metal can be sputtered on the foil, or alternatively deposited on the foil using chemical or physical vapor deposited techniques. The alloy 310 should at least be applied to two sides 301 and 302 of the foil 300 where the interface is to be formed, but can be deposited on all sides of the foil 300 as illustrated.

Referring back to FIG. 1, as before the embedded structure 300 of FIG. 3 is then applied in between the two surfaces 110 and 120, in solid or substantially solid form. As discussed earlier, the heat dissipated from the computing environment, or the surfaces 110 and 120, during normal device operation will melt the alloy and form a gap free, bubble free interface as was discussed in conjunction with FIG. 1.

It should be noted that the foil 300 and or the structure 200 of FIGS. 2 and 3 can be provided easily in the size and shape of the chips to be cooled when desired. Once in place between the chips and the heat sink, when the chips are powered up, for example, the heated chips alone can provide the desired melting of the low-melting alloy which then bridges the gap between the chips and the heat sinks.

Different structural needs may be present leading to the use of one form of the structure or foil over another. In other words, the wire mesh structure of FIG. 2 and the foil structure of FIG. 3 can be used selectively and for different reasons. For example, the wire-mesh backed interfacial structure 200 of FIG. 2 is beneficial when there is high compressive force between the heat sink and the chip. The interfacial gap is controlled to be twice the wire thickness. Thus, there is less forcing out of the molten interface alloy by the compressive forces. The structure 200, can in this way be fabricated to provide more or less compression as selectively desired. The foil 300 of FIG. 3 can be used in instances where greater flexibility of the foil is desired as the foil can be fabricated in a less rigid manner than the structure 200 of FIG. 2 for apparent reasons.

In this manner the present invention circumvents the problem of poor wetting inherent in the low-melting temperature alloy interfaces between microprocessor chips and heat sinks by teaching a structure and method of bridging the gap between a chip and a heat sink by a low-melting alloy even when the low-melting alloy does not wet the chip and the heat sink.

While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described. 

1. An apparatus for forming a thermal interface comprising of a first structure embedded in a low melting alloy, said melting alloy not being able to readily react with the material of said first structure, said embedded first structure and said molten alloy being applicable to an interface in solid form without forming a bonded interface but forming such bonded thermal interface during device operation and by application of device dissipated heat.
 2. The apparatus of claim 1, wherein said first structure is apertured.
 3. The apparatus of claim 2, wherein said first structure is metallized.
 4. The apparatus of claim 3, wherein said first structure is a wire mesh.
 5. The apparatus of claim 3, wherein said first structure is rigid and its rigidity is selectively proportional to level of surface apertures.
 6. The apparatus of claim 1, wherein said structure is used to provide a thermal joint between a heat sink and a chip in a semiconductor device.
 7. The apparatus of claim 6, wherein the size of said structure can be selectively altered according to size of said chip.
 8. The apparatus of claim 7, wherein said alloy embedded structure disposed in solid form between said chip and said heat sink and the operating of said sink will melt said alloy and form desired thermal interface.
 9. The apparatus of claim 3, wherein said alloy is applied to all sides of said structure.
 10. The apparatus of claim 9, wherein said alloy is applied to all sides of said structure including in between apertures.
 11. The apparatus of claim 9, wherein thickness of said alloy embedded structure depends on amount of alloy applied to said structure sides.
 12. The apparatus of claim 9, wherein said alloy is sputtered on all sides of said structure.
 13. The apparatus of claim 9, wherein said alloy is at least sputtered to two sides of said structure where thermal interface is to be formed.
 14. The apparatus of claim 9, wherein said alloy is deposited on said structure using a physical or chemical vapor deposition technique.
 15. An apparatus for forming a thermal joint comprising of a metal foil embedded in a low melting alloy, said melting alloy not being able to readily react with said foil, said embedded foil and said molten alloy being applicable in solid form without forming a bonded interface and only forming such interface after application of heat of dissipated heat formed during normal device operation.
 16. The apparatus of claim 15, wherein said foil is used to provide a thermal joint between a heat sink and a chip in a semiconductor device.
 17. The apparatus of claim 16, wherein the size of said foil can be selectively altered according to size of said chip.
 18. The apparatus of claim 17, wherein said alloy embedded foil is disposed in solid form between said chip and said heat sink and the operating of said sink will melt said alloy and form desired thermal interface.
 19. The apparatus of claim 18, wherein said alloy is applied to all sides of said foil.
 20. In a semiconductor device, a method for forming a thermal interface in a computing environment comprising embedding a metal structure in a low melting alloy, said melting alloy not being able to readily react with said structure, disposing said embedded structure in solid form between a heat sink and a chip such a thermal interface and bond is only formed after heat dissipated from said chip melt said alloy during normal device operation. 